Modeling the role of riverine organic matter in hypoxia formation within the coastal transition zone off the Pearl River Estuary

Abstract

Globally expanding hypoxia in estuaries and coastal oceans has largely been attributed to the elevated river nutrient inputs, whereas the role of river‐delivered terrestrial organic matter (OMterr) in hypoxia formation has been less investigated. This study uses a coupled physical‐biogeochemical model and observations to investigate how OMterr directly (via remineralization) and indirectly (via the nutrients released from OMterr remineralization) promotes hypoxia development in the coastal transition zone off the Pearl River Estuary. Results show that direct contribution of OMterr remineralization to total oxygen consumption by terrestrial and marine organic matter negatively correlates with salinity, decreasing from over 60% in the upper estuary to nearly 0% in the far reaches of the river plume, and is higher in the upstream (average 30%) than the downstream region (average 18%). Nevertheless, the nutrients released from OMterr remineralization greatly sustain an indirect contribution to oxygen depletion and hypoxia formation downstream. The increasing relative importance of indirect over the direct effect of OMterr to hypoxia along the plume path is a combined result of the wind‐driven eastward shelf current and the OMterr‐released nutrients being advected farther downstream than the sinking OMterr. This highlights that without including the indirect effect of OMterr may underestimate the role of OMterr in hypoxia formation in aquatic systems. Examinations of the hypoxia response to varying riverine loads further suggest that reducing the nutrient and OMterr loads is required for hypoxia mitigation in the upstream region while reducing the nutrient load alone is more effective in mitigating hypoxia in the downstream.

Coastal Upwelling Combined With the River Plume Regulates Hypoxia in the Changjiang Estuary and Adjacent Inner East China Sea Shelf

“…Life‐sustaining dissolved oxygen (DO) plays an essential role in elemental biogeochemical cycles (Cai et al., 2017; Hietanen et al., 2012; Rabalais et al., 2014) and is crucial for the survival of aquatic organisms (Breitburg et al., 2018; Diaz & Rosenberg, 2008; Levin et al., 2009). Due to the enhanced natural and anthropogenic perturbations, hypoxia has been frequently observed in coastal marine ecosystems, especially in river‐dominated regions (Hetland & DiMarco, 2008; Howarth et al., 2011; D. Li et al., 2020; Pitcher et al., 2021; Rabouille et al., 2008; Yu et al., 2020). Hypoxic zones can deteriorate ecosystem services, such as the sustainability of fisheries habitats and biodiversity (Breitburg et al., 2018; Diaz & Rosenberg, 2008; J. Zhang et al., 2010).…”

Distinct interaction effects of warming and anthropogenic input on diatoms and dinoflagellates in an urbanized estuarine ecosystem

“…Industrial waste, agricultural fertilizer, animal excretions and degradation of organic nitrogenous matter are the common sources of nutrient loadings (Bouwman et al, 2013; Li, Nwankwegu, et al, 2020). Serious environmental issues can result from these extra loading of nutrients, such as harmful algal blooms and hypoxia (Anderson et al, 2002; Yu et al, 2020). Many species of dinoflagellates can produce toxin and their proliferation can lead to mass killing of marine organisms and even health risks to terrestrial life if the poisoned prey is consumed.…”

Machine Learning Based Long‐Term Water Quality in the Turbid Pearl River Estuary, China

“…As a unique pigment essential for photosynthesis, Chl‐a has been widely used as a proxy for phytoplankton biomass (Hirawake, 2005; Jasprica & Carić, 1997). Anomalously high Chl‐a concentration is likely related to eutrophication and algal bloom (Shutler et al., 2012; Tomlinson et al., 2009) and is one of important conditions for the formation of hypoxia (D. Li et al., 2020; Yu et al., 2020). These phenomena have been frequently observed around the world (Breitburg et al., 2018; Klemas, 2012; Sha et al., 2021; Zheng & DiGiacomo, 2020).…”